The behavioral manipulation of insect pests for their management, as an alternative to broad-spectrum insecticides, has been investigated for many years.
In addition to the development of resistance against insecticides by the target organism, broad-spectrum insecticides also have negative impacts on natural enemies of the pest insect, on pollinators and on other non-target organisms. Therefore, there is an increased interest in the behavioral manipulation of insect pests for their management as an alternative to broad-spectrum insecticides. Of particular interest are compounds that do not exhibit substantial toxicity or demonstrate some degree of selectivity towards a pest insect and not toward natural enemies, pollinators or the environment. In practice, manipulation may be achieved through the use of stimuli that either enhance or inhibit a particular behavior and ultimately change its expression. Many natural plant defensive chemicals discourage insect herbivory, for example, by deterring feeding and oviposition or by impairing larval growth, rather than by killing insects.
The choice of a stimulus for behavioral manipulation is usually dependent upon a number of factors including accessibility, reproducibility, specificity and practicality (Foster and Marris 1997). Various short- or long-range stimuli, involved in behavioral manipulation of insects, are perceived through contact chemoreceptors or olfactory receptors, respectively. These stimuli can either stimulate feeding or oviposition, keeping the insect at the host plant, or inhibit those behaviors, resulting in the insect abandoning the plant. Examples of feeding stimulants often include carbohydrates, proteins, or fats (Ave 1995) that are ubiquitous in plants, whereas oviposition stimulants can be highly species-specific. Feeding stimulants can be used in conjunction with toxins in “attract and kill” strategies (Ave 1995), occasionally employed in crop protection. A deterrent can be applied to a host plant to prevent feeding or oviposition. Therefore, deterrents may have potential value in crop protection, in combination with other strategies such as “attract and kill” (Jermy 1965; Munakata 1970).
Insect feeding deterrents can be found among all the major classes of plant secondary metabolites—alkaloids, phenolics and terpenoids (Frazier 1986). Especially well studied in this group are the triterpenes such as the limonoids from the neem (Azadirachta indica) and chinaberry (Melia azedarach) trees and from Citrus species and the diterpenes including the clerodanes and the abietanes (Isman 2002). Apart from terpenes, another important class of compounds involved in defense of the plant against herbivores and pathogens, as well as in attracting pollinators, are the compounds derived from aromatic amino acids—phenylpropanoids (Wildman 2006).
Eugenol is a volatile member of the phenylpropanoid class of compounds from essential oils of many spices, particularly clove (Dewick 2002). Cloves are useful in the home as moth deterrents and the main odorant from cloves, eugenol, has been reported to be perceived as a long-range stimulus by several lepidopterans (Topazzini et al. 1990). One problem with phenylpropanoids such as eugenol and compounds with a cinnamyl framework is that they can produce toxic metabolites after benzylic/allylic oxidation by certain cytochrome P450 enzymes (Dewick 2002).
Several polyphenolic compounds are also known for their toxic/insecticidal effects (Kim and Ahn 2001; Schneider et al. 2000; Khambay et al. 1999; Harborne 1989). Flavonoids isolated from Annona squamosa (Kotkar et al. 2002), Ricinus communis (Upasani 2003) and Calotropis procera (Salunke et al et al. 2005), are toxic to the pulse beetle. Callosobruchus chinensis and R. communis also caused oviposition deterrent and ovicidal affects in addition to toxicity. Larvicidal activity of lignans, leptostachyol acetate and analogues from the roots of Phryma leptostachya have been reported against three mosquito species (Culex pipiens pallens, Aedes aegypti, and Ocheratatos togoi) (Park et al et al. 2005).
Compounds derived from aromatic amino acids, such as some phenolics, have been reported to be involved in defense of the plant against herbivores and pathogens, as well as in attracting pollinators. For example phenol derivatives such as guaiacol (1-hydroxy-2-methoxybenzene), 1,2-dimethoxybenzene, 1-ethoxy-2-methoxybenzene, 1-propoxy-2-methoxybenzene, eugenol and isoeugenol, occur in smoke (Guillen and Manzanos 2005; Murugan et al et al. 2006) and are reported to have insect-repellent and insecticidal activities (Murugan et al et al. 2006). Furthermore, smoke phenolics taste and smell pleasantly (to humans) (Guillen and Manzanos 2005) and may have antioxidant activity (Bortolomeazzi, et al. 2006). Eugenol (2-methoxy-4-(2-propenyl)phenol), is found in herbs (such as basil, Ocimum suave (Wild.)) and has been reported to have activity against grain beetles as a toxicant and deterrent (Obeng-Ofor and Reichmuth 1997). Other benzene derivatives, such as benzyl alcohol, benzonitrile, phenylethanol, 4-methyl phenol, 4-ethylphenol, 2-methylphenol and benzaldehyde are reported components of human odor that malaria mosquitoes respond to (Hallem et al. 2004; Meijerink et al. 2000).
Widely distributed, the cabbage looper Trichoplusia ni is considered an important field and greenhouse pest in vegetable crop production. This species is a generalist and attacks a variety of crops including lettuce, beets, turnip, spinach, brussel sprouts, peas, celery, tomatoes, rape, tobacco, certain ornamentals, many weedy plants, as well as cruciferous plants. Moths emerge in the spring and use two mate-finding strategies (Landolt and Heath 1990). One strategy involves male attraction to the female-produced sex pheromone which includes the major component Z-7-dodecenyl acetate (Berger 1966) and several other structurally related compounds (Bjostad et al. 1984). The other strategy involves female attraction to the male pheromone composed of the major component S-(+)-linalool, as well as p-cresol and m-cresol (Heath et al. 1992). The amount of pheromone released by the male has been reported to be affected by the cabbage odour.
The mated females deposit dome-shaped, pale green eggs singly on the host-plants, chiefly at night. After hatching, the destructive larval stage reaches full development in two to four weeks; pupation then occurs and in almost 10 days the new adults emerge. In general, the larval stages damage the crop. The first two larval stages feed on the lower side of the leaf, eating through the upper epidermis, leaving “windows” in the leaf. Older larvae chew larger holes in the leaves, often resulting in extensive damage to leaves. Although this pest generally damages leaves, damage has been reported on watermelon rinds and on flowers of various host plants. Three or more generations are generally produced each season, depending on the latitude (Davidson and Lyon 1979).
The loopers overwinter in the pupal stage, the pupae enclosed in flimsy silken cocoons attached to the food plants or to nearby objects. Cabbage loopers do not generally overwinter in Canada and migrate in from the south. However, they can overwinter in greenhouses.
Chemosensory input from contact chemosensilla present on the tarsi, antennae, and other parts of the body, such as the ovipositor, affects feeding and oviposition behaviors in cabbage looper as well as other phytophagous insects. Based on the sensory information received, an insect can chose a proper feeding or an oviposition site. Neonates of many species including cabbage looper are incapable of locating a new host and are dependent on the host plant location “skills” of their mothers (Feeny et al. 1983). Therefore, the site of emergence is of importance to the larvae of many lepidopteran species (Restraits 1966).
T. ni has developed resistance to a number of commercial insecticides, including early generation insecticides such as DDT, carbaryl, parathion, (McEwen 1956) as well as more modern insecticides such as methomyl and Bt (Bacillus thuringiensis toxin), a widely used benign and specific insecticide against moth pests that are in the larval stages (Wang et al. 2007).